1. Field of the Invention
The present invention relates to a transmission gearbox, and more particularly to an improved structure of transmission gearbox for use in a motor vehicle as to eliminate the problem of difficulty in adjusting the presence of a backlash of the transmission bevel gear in the prior art and achieve the purposes of easy maintenance and assembly and reducing the number of parts within the gearbox.
2. Description of Related Art
FIG. 1 shows a transmission gearbox for use in a motor vehicle according to the prior art. According to this design, the transmission gearbox 10 comprises a driven shaft 11 adapted to receive the driving force of the engine through a belt transmission mechanism (not shown), a transmission gear 111, and a linking shaft 12 adjacent to the driven shaft 11. The linking shaft 12 has mounted thereon in proper order a first gear 121, a back sliding block 122, a back bevel gear 124, a second gear 125, a third gear 126, a forward bevel gear 127, and a fourth gear 128. The first gear 121 is meshed with the transmission gear 111. The back sliding block 122 and the back bevel gear 124 each have a coupling hole 123 or 123′. The back sliding block 122 has a toothed portion (not shown) meshed with the linking shaft 12 such that the back bevel gear 124 and the forward bevel gear 127 and the fourth gear 128 run idle during rotation of the linking shaft 12.
Further, a main shaft 13 is provided at the other side of the linking shaft 12. The main shaft 13 has mounted thereon in proper order a low-range gear 131, a corresponding sliding block 132, a forward gear 133, and a transmission gear 134. The low-range gear 131, the sliding block 132 and the forward gear 133 each have a coupling hole 123 or 123′. The sliding block 132 has a toothed portion (not shown) meshed with the main shaft 13 such that the second gear 125 at the linking shaft 12 meshes with the forward gear 133 at the main shaft for synchronous rotation and the low-range gear 131 and the forward gear 133 run idle on the main shaft 13 before connection of the sliding block 132 to the coupling hole 123 of the forward gear 133.
Further, a first push rod 141 and a second push rod 143 are respectively fastened to the coupling hole 123 of the back sliding block 122 and the coupling hole 123′ of the sliding block 132. The first push rod 141 and the second push rod 143 are respectively mounted on a shaft 14, each having the other end terminating into a respective tip 142 or 144 that is respectively inserted into a respective sliding groove 151 or 152 at a gearshift hub 15 for moving a gearshift lever 16 into the desired gearshift position.
Further, the forward bevel gear 127 at the linking shaft 12 is meshed with a transmission bevel gear 21 at the output shaft 20.
When shifting the gearshift lever 16 to the back gearshift position, the drive gear 161 at the gearshift lever 16 moves a driven gear 153 at the gearshift hub 15. At this time, the first push rod 141 pushes the back sliding block 122 into the coupling hole 123′ of the back bevel gear 124, for enabling the driving force to be transferred from the transmission gear 111 at the driven shaft 11 to the linking shaft 12. Because the back bevel gear 124 is sleeved onto the linking shaft 12. Rotating the linking shaft 12 does not cause the back bevel gear 124 to rotate. At this time, the back sliding block 122 drives the linking shaft 12 to rotate, and therefore the engine driving force is transferred through the back bevel gear 124 to the transmission bevel gear 21 and the output shaft 20 and then to the rear wheels of the motor vehicle for backward movement.
According to this design, the transmission bevel gear 21 at the output shaft 20 is adapted to mesh with the back bevel gear 124 and the forward bevel gear 127. During gear transmission between the transmission bevel gear 21 and the back bevel gear 124/forward bevel gear 127, a noise is produced due to the presence of a backlash. Further, the processing precision requirement of these bevel gears is critical. Loose engagement between the transmission bevel gear 21 and the back bevel gear 124/forward bevel gear 127 produces a high noise during transmission.
FIG. 2 shows another structure of transmission gearbox according to the prior art. According to this design, the transmission gearbox 30 comprises a gearbox body 31 and a gearbox cover 32. The gearbox body 31 and the gearbox cover 32 define a space for accommodating an output shaft set 33, which is comprised of an output shaft 34, a bearing 35, and a transmission bevel gear 36. A C-shaped clamp 37 is installed in the transmission gearbox to secure the bearing 35 to the inside of the transmission gearbox between the gearbox body 31 and the gearbox cover 32. The positioning precision of the C-shaped clamp 37 is also critical. If the C-shaped clamp 37 is not accurately installed in position between the gearbox body 31 and the gearbox cover 32, a big backlash will be produced at the transmission bevel gear 36, thereby resulting in a high noise during transmission operation of the transmission gearbox.